User manual

Advanced Function Programming
3-21
SIMATIC 505–2557 Installation and Operation Guide
Digital filtering time is the settling time to within 1 LSB of the
analog-to-digital converter on the module. (Often digital filtering is specified
as a time constant in milliseconds. W ith a time constant specification, it will
take the input 4 to 5 time constants to reach 99% of the final value.)
The value entered is the actual settling time.
NOTE: In the SIMA TIC 505–2557 module, the value used in digital filtering
is not a time constant but is the settling time for the system to reach the full
resolution of the analog-to-digital converter (ADC).
When filtering is enabled, the actual resolution of the module is a full 16
bits. The filtering function performs a dithering operation for the least
significant bits.
Default Filter Settling T ime If the digital filtering bits are enabled via
the WY register and the Y32 output and no settling time values are written
to the module, then the default digital filter settling time of 250 msec is
automatically used.
Filtering and Alarms If filtering is enabled, then the filtered data will
be used for alarm comparisons; that is, the data will first pass through the
digital filter and its associated settling time and then be compared to any
low or high alarm setpoint. This prevents alarm conditions that are
attributable to noise.
Changing the Settling T ime When new filter data is written to the
module, the microcomputer must recompute the filter time constants. This
operation takes 25 msec and no new data is written to the controller during
this time.
Numerical Range Values loaded into the module for digital filtering are
expressed as 16-bit unsigned integers 0 to 65535 in units of milliseconds.
NOTE: Signed integers will be interpreted as unsigned values.
Digital Filtering